The present invention relates to a controller used for a permanent magnet synchronous motor and an electric vehicle controller using the controller.
A permanent magnet synchronous motor controller is disclosed, for example, in Japanese Patent Application Laid-Open No.6-315201. In a case where control of a permanent magnet synchronous motor is started when the permanent magnet synchronous motor is being rotated at a high speed, or in a case where switching elements of an inverter are in an OFF state under a state in which a rotating speed of the permanent magnet synchronous motor exceeds a controllable range, the input side of the inverter is in a direct current high voltage due to electric generating operation of the permanent magnet synchronous motor. If a battery and the inverter are connected by a relay circuit under such a condition in order to start or restart the control of the permanent magnet synchronous motor, overcharge of the battery will occur. Therefore, in the above prior art, a contactor for breaking the connection between the inverter and the permanent magnet synchronous motor is provided, and the connection between the inverter and the permanent magnet synchronous motor is broken when the power supply to a pre-drive circuit for driving switching of the inverter is cut in order to avoid the high voltage condition in the input side of the inverter due to electric generating operation of the permanent magnet synchronous motor.
In the prior art of Japanese Patent Application Laid-Open No.6-315201, there is a problem in that the cost is high and the weight is increased since the contactor is used.
Further, Japanese Patent Application Laid-Open No.1-190286 discloses a technology where stop maintaining control is performed by 3-phase short circuiting of at least two of three switching elements connected to a negative pole side of a direct current power source among six switching elements of an inverter at the same time to generate a braking force in a permanent magnet synchronous motor.
The technology disclosed in Japanese Patent Application Laid-Open No.1-190286 is for control during stopping of a vehicle and not for control during running operation. Therefore, there still remains a problem in that when the 3-phase short-circuit is performed while the permanent magnet synchronous motor is being rotated at a high speed, a large current flows from the motor to the switching elements of the inverter at an initial stage of the short circuit (transiently). If switching elements having a large maximum allowable current are employed, the cost is increased.
Here, explanation will be made below on a case where the 3-phase short circuit is required in a permanent magnet synchronous motor controller performing ON-OFF control operation.
The control of a 3-phase short circuit can be performed by outputting signals to turn on all switching elements 20, 22, 24 connected to a positive pole side of a direct current power source and to turn off all switching elements 21, 23, 25 connected to a negative pole side of the direct current power source from a controller 11. By the control, currents of 3-phase terminals of a motor 1 flow through the switching elements 20, 22, 24 and the corresponding diodes, and consequently the 3-phase terminals of the motor 1 are physically coupled.
Therein, the 3-phase short circuit can be attained by turning on the switching elements 21, 23, 25 and turning off the switching elements 20, 22, 24.
However, when the 3-phase short circuit is simply performed, there occurs the following problem. FIG. 10 is a series of charts explaining motor currents and motor torque when 3-phase short circuit control is performed. The figure shows motor currents and motor torque when 3-phase short circuit control is performed by turning on the switching elements 20, 22, 24 at a time at "time 0" without any additional operation while the switched elements 21, 23, 25 are being kept switching off. FIG. 10 (a), FIG. 10 (b), FIG. 10 (c) and FIG. 10 (d) show U-phase current, V-phase current, W-phase current and torque characteristic, respectively.
The characteristics shown here illustrate that the U-phase current flows are largely shifted to the positive side and the V-phase current and the W-phase current flows are shifted to the negative side in the initial period, and then all the currents in the U-phase, the V-phase and the W-phase are settled to sinusoidal waves having a center of a 0 (zero) current value after sufficient time elapses. During that time period, torque fluctuation in connection to the fluctuation also occurs in the motor torque. The characteristic of the shift in the motor currents at the starting of the 3-phase short circuit control changes depending on a position of the magnetic pole of the permanent magnet at a starting point of the 3-phase short circuit as the V-phase current is shifted most or the W-phase current is shifted most, or the current is shifted to the positive side or the current is shifted to the negative side.
The shifting phenomenon in the conventional technology will be further explained below, referring to FIG. 11 and FIG. 4. FIGS. 11a and 11b are diagrams explaining a motor equivalent circuit of FIG. 10. FIGS. 4a and 4b are charts explaining motor current time characteristics at the short circuit control.
That is, FIG. 11 (a) is a diagram showing an equivalent circuit of the permanent magnet synchronous motor while performing the 3-phase short circuit. In the figure, the reference character Ra is an armature winding resistance, the reference character La is an armature winding self-inductance, and eu, ev and ew are induced voltages by the permanent magnet. The current flowing in the equivalent circuit of FIG. 11 (a) (called a steady-state current component) is shown in FIG. 4 (a). FIG. 11 (b) is a diagram showing an equivalent circuit of the permanent magnet synchronous motor subtracting the induced voltage by the permanent magnet. The current flowing in the equivalent circuit of FIG. 11 (b) (called a transient current component) is shown in FIG. 4 (b). The current while performing the 3-phase short circuit can be expressed as the sum of the steady-state current component of FIG. 4 (a) and the transient current component of FIG. 4 (b).
The steady-state current component of FIG. 4 (a) is a current characteristic (sinusoidal wave having an amplitude i1) of one phase (U-phase) when sufficient time elapses after starting the 3-phase short circuit. The induced voltage eu is generated with a phase leading the current by nearly 90.degree. if the frequency of the current is very high, though it is not shown. The current shift at the starting of the 3-phase short circuit is varied depending on what phase of the steady-state current component the timing of the starting the 3-phase short circuit is started at.
That is, referring to the steady-state current component of FIG. 4 (a), in a case where the 3-phase short circuit control is started at a timing of a phase at time point (1), the current begins to flow without shifting to the positive side nor to the negative side. In a case where the 3-phase short circuit is started at a timing of a phase at time point (2), the current begins to flow with shifting to the switching element connected to the negative side. In a case where the 3-phase short circuit is started at a timing of a phase at time point (3) (in the case corresponding to the figure of FIG. 10 (a)), the current begins to flow with shifting to the switching element connected to the positive side.
This is because the current before starting the 3-phase short circuit is 0 (zero), and the current is started to flow from 0 (zero) to the negative side when the 3-phase short circuit is started at a timing of a phase at time point (2), and then the current flows twice as much as the amplitude of the sinusoidal wave. Similarly, the current is started to flow from 0 (zero) to the positive side when the 3-phase short circuit is started at a timing of a phase at time point (3), and then the current flows twice as much as the amplitude of the sinusoidal wave. Therefore, when the 3-phase short circuit is started at a timing of a phase at time point (1), there is no shift in one phase (U-phase). However, since the V-phase and the W-phase are behind the U-phase by 120.degree. and 240.degree. respectively, it is impossible to eliminate all shifts in the three-phase.
On the other hand, the transient current component of FIG. 4 (b) indicates the shift of current at starting the 3-phase short circuit. The value iu' is a shifting current component (3-phase short circuit transient current component) of the U-phase, and decays with a time constant .tau. expressed by the following equation. EQU .tau.=La/Ra (1)
Similarly, 3-phase short circuit transient components of the V-phase and the W-phase decay with a time constant .tau..
The 3-phase short circuit transient current component iu' can be expressed as Equation (2). EQU iu'=iu'0.multidot.exp(-1/r) (2)
The initial value iu'0 of the decay curve is a reversed sign value of the phase current value at the starting of the 3-phase short circuit in the steady-state current component of FIG. 4 (a). That is, the initial value iu'0 is -i1 when the 3-phase short circuit is started at the timing of the phase (2), and the initial value iu'0 is i1 when the 3-phase short circuit is started at the timing of the phase (3). Similarly, each of the initial values of the 3-phase short circuit transient current components for the V-phase and the W-phase is a reversed sign value of the corresponding phase current value at starting of the 3-phase short circuit in the steady-state current component.
When the current flowing at the starting of the 3-phase short circuit exceeds a maximum allowable current value of the switching element (IGBT or the like), there is a possibility that the switching element is broken. Further, it may be considered that the motor torque is fluctuated as shown in FIG. 10 (d) to adversely affect the driving system (gears or the like).